High-speed communication systems typically communicate with each other by sending serial bit streams of data between transmitters and receivers. These bit streams are usually binary coded pulse signals represented by zeros and ones which may be electrical voltages or optical signals derived from the signals created by the transmitters and which pulse coded signals are applied to a transmission facility connecting the transmitters with the receivers. The receivers decode the received pulse code signal data to obtain the information therein.
If a receiver receives pulse code signals that have been deformed by errors occurring in the transmission facility or if the receiver improperly decodes the received pulse coded signals, the effect is that bit errors may occur in the communications thereby resulting in wrong information being received by the receiver. Thus, designers, engineers, installers and maintenance personal need to evaluate the stream of pulse coded signals, oftentimes called binary pulse bit streams, to monitor system performance and to help in diagnosing system problems. It is typical to monitor the quality of such bit streams by using a sampling oscilloscope.
In the monitoring operation, the binary pulse bit stream and a trigger input in the form of a clock signal having a repetition rate identical to the repetition rate of the binary pulses in the bit stream and synchronous therewith are applied to the inputs of the sampling oscilloscope. Samples of the voltage levels of the binary pulses of the bit stream are taken at various time offsets from the repetitive trigger input and are plotted as sample points on the display of the oscilloscope. Voltage samples are continuously taken of the bit stream and added to the sampling oscilloscope in combination with the older sample points which continue to exist on the sampling oscilloscope display. Over a relatively short period of time, hundreds or thousands of the sample points on the sampling oscilloscope display plot the possible voltage distributions at each time offset from the trigger input. By sweeping all time offsets in the range of interest, a diagram appears on the sampling oscilloscope display which reveals the quality or characteristics of the measured high-speed bit stream. This type of diagram, oftentimes called an “eye” diagram, is often used to view high speed binary pulse bit streams during the various development, installation and maintenance phases of high-speed communications systems.
A problem exists in using sampling oscilloscopes in this manner to measure the quality of high-speed communication systems. As the bit stream data rate increases, the bandwidth of the sampling oscilloscopes needed to create the eye diagrams increases proportionally thereby resulting in a higher cost. Another problem arises in that as the cost of the sampling oscilloscopes increases and due to the design issues of super high-speed systems, the present sampling methods takes samples at a relatively few of all the possible edges of the bit stream pulses thereby limiting the current effective sampling rate to the range of one hundred to two hundred thousand samples per second.
Another type of apparatus has been developed to create an eye diagram for statistically measuring the quality of high speed binary pulse bit streams. A problem exists with this apparatus in that it requires that multiple histrograms be created and combined to establish an eye diagram and that two histograms are required to be created to make an eye diagram. Subtractions from neighboring vertical elements in the first histrogram are required to make the second histrogram which is what is used to make the eye diagram. If the data count of the user signal passing through the device under test changes drastically between the times the two vertical elements are taken, noise can be added to the subtracted results causing noise in the eye diagram. Accordingly, a need exists in the art for apparatus and a method for creating a noise free single histrogram actively determining an eye diagram for measuring the characteristics of high-speed binary pulse bit streams used to transfer information and data between communications systems.